A fuel cell, as used herein, provides for the direct production of electrical energy through an electrochemical reaction involving fuel and gas reactants, which may be typically hydrogen and oxygen. A single cell comprises an anode structure, cathode structure, and an electrolyte separating the electrodes. A particular form of electrolyte is a polymer electrolyte membrane for ionic transport between the electrodes.
One of the primary challenges in attaining an optimal performance of polymer electrolyte membrane fuel cells is attaining effective hydration of the ionomeric membrane structure. At less than ideal hydration levels, the water content of the ionomer drops with a concomitant decrease in the ionic conductivity. In the case of fuel cells, the kinetics of the oxygen reduction reaction (ORR) at the cathode are also adversely affected by a decrease in water content.
Maintaining a high hydration level at all times throughout the membrane/electrode assembly (MEA) is difficult in fuel cells for a number of reasons. For example, at low current densities or at open circuit, when little liquid water is produced by the ORR, the MEA hydration level will tend to drop even when the reactant gases are at saturated water vapor conditions because the water uptake of perfluorosulfonate membranes is less when a membrane is vapor equilibrated rather than liquid equilibrated. On the other hand, at higher current densities, the electro-osmotic drag of water with the ionic flux from the anode to the cathode across the membrane tends to dry out the anode. An additional problem is that the cathode then tends to flood because of the ORR generated water as well as the water dragged across the membrane. In general, it is difficult to maintain optimum hydration levels under all operating conditions.
Under certain conditions, the water produced in the ORR reaction is sufficient to maintain adequate hydration. It is still difficult to maintain adequate performance outside of a particular operating envelope, which typically does not include low current densities, typical operating temperatures (80-90.degree. C.), or near ambient reactant pressures, without humidification of at least the anode region. Some approaches to humidification systems or to humidification control entail some means of introducing humidification plates within the individual cells. In a further evolution of this approach, International Fuel Cells (IFC, South Windsor, Conn.) uses water permeable bipolar plates to recover the water from the cathode plenums and directly humidify an anode plenum of an adjacent cell.
In most other polymer electrolyte fuel cell technologies, the water is supplied to the cell by humidifying the reactant gas streams. In several types of commercial fuel cell stacks, the gas streams are humidified by flowing the reactants through a humidity exchanger. The exchanger and its associated control system tend to increase system size and complexity.
Control of the stack is also complicated if the cells are humidified via the reactant streams because the two systems then become coupled. Often, the optimal hydration level may not match the most efficient reactant flowrate, and it may be difficult, for example, to switch-off the cathode humidification if the cell starts to flood, or to increase the humidification level at low current densities when the cell starts to dry out as less water is produced by the ORR. This lack of water is further exacerbated by the tendency of the membrane to take up less water when only vapor humidified. For such reasons, it is difficult to provide optimal (and not excessive) hydration over all current densities with a particular set-up.
One approach to alleviate these difficulties is to supply sufficient liquid water directly to the membrane in some manner to de-couple the humidification and reactant supply subsystems and maintain the ionomer at the liquid-equilibrated hydration level at all times. In general, the best way of decoupling hydration from the other subsystems and assure liquid-equilibrated hydration levels is to introduce liquid water directly to the MEA, as has been done previously by wicking from the periphery through the ionomeric membrane, or to inject water from the periphery through miniature tubes formed in the membrane. In the former case, Watanabe et al. (J. Electrochem. Soc., 140, 3190 (1993)) introduces liquid water to the membrane directly through a supply reservoir at the periphery of the electrode. Since the standard perfluorosulfonate membranes do not wick water particularly well, Watanabe et al. teach a composite layer within the membrane that consists of the ionomer for ionic conductivity and a wicking material to facilitate the transport of the water. Performance improvements over reactant humidification are demonstrated in small cells. In the other liquid hydration approach, Lynntech, Inc. (College Station, Tex.) impresses miniature channels into conventional perfluoroionomer membranes and injects water from the edge through the tubes thus formed. In these configurations it may be difficult to wick or pump the water a substantial distance. The membranes need to be relatively thick and currently available membranes and MEAs can not be directly utilized.
Accordingly, it is an object of the present invention to supply liquid water to the polymer electrolyte membrane of a fuel cell using generally available fuel cell components and without affecting the active area of the components.
Another object of the present invention is to provide a relatively simple membrane humidification system for use with a fuel cell.
One other object of the present invention is to uniformly distribute liquid water to a surface of a membrane.
Yet another object of the present invention is to decouple control of membrane humidification from control of reactant flow rate.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.